The attachment of sugars to naturally-occurring and/or synthetic small molecules can dramatically influence the corresponding mechanism, pharmacodynamics, pharmacokinetics and even patent life of the parental structure. Yet, due to the technical challenges associated with conventional glycosylation strategies, the application of glycosylation in the context of drug discovery and/or development remains underexplored. An overarching aim of this program has been to develop simple and user-friendly chemoenzymatic methods for the differential glycosylation of target scaffolds to ultimately enable such drug discovery/development exploration. Toward this goal, the first phase of this study (years 1-5) led to the key proof of concept for chemoenzymatic glycorandomization (a one pot, three enzyme strategy capable of activating and attaching free monosaccharides to complex natural product scaffolds) and also provided fundamental information regarding two critical, but poorly understood, enzyme classes (anomeric sugar kinases and sugar-1-phosphate nucleotidylyltransferases). The second phase of this project (years 6-10) dramatically expanded the attempted application of this first generation system toward a range of diverse scaffolds, leading to success in many cases but also exposing key limitations of the platform. Importantly, the work conducted during the second phase also led to fundamental new knowledge regarding glycosyltransferase (GT)-catalyzed reactions that serves as the basis from which to launch the third phase of this study. Specifically, the proposed third phase of this study (years 11-15) takes advantage of our recent abilities to evolve highly permissive GTs and also drive GT-catalyzed reactions in reverse to enable next generation single enzyme or dual enzyme transglycosylation strategies for differential glycosylation of an array of structurally-diverse scaffolds (including natural product-based or synthetic, glycosylated or non-glycosylated, parental scaffolds). We propose to take full advantage of the current state of the art to: i) narrow the gaps of knowledge in understanding functional GT structure-activity-relationships; ii) specifically develop a range o catalysts for the production of novel sugar nucleotides (anticipated to be of broad use to the glycobiology community); iii) develop a range of catalysts for the differential glycosylation of a key set of structurally diverse anti-infective and anticancer scaffolds; and iv)exploit the corresponding differentially glycosylated scaffolds as a novel source for the discovery of new antiinfective and anticancer leads.